Where to find deb file for Blender 2.78c
I ran into the same problem last night. Fortunately, I found a copy of the original 2.78c package in my apt cache. Here is my solution:


purge any existing blender installation (via apt-get) sudo apt-get purge blender
add the ppa Thomas Schiex sudo add-apt-repository ppa:thomas-schiex/blender;
sudo apt-get update; (Do not install blender from there directly. It would install 2.79 again, which we do not want here!)
download the 2.78c version of blender manually from here: https://drive.google.com/open?id=0B4SAG_u19QW3RnhGYk5tU3kxekE (I would prefer a more official link to a working package here, but I found non.)
install the package via gdebi (assuming you downloaded to /home/$USER/Downloads): sudo gdebi /home/$USER/Downloads/blender_2.78c~1487946787-0thomas~xenial1_amd64.deb (dependencies are resolved from ppa Thomas Schiex)
Finished! Enjoy your 2.78c version of blender again.
to prevent from automatic updates, set the package to be unmodifiable: sudo apt-mark hold blender